Add a default q/p to the PrPixel track states

Implements a default q/p in PrPixel track states, as was done in Run1/2 for Velo tracks, to avoid NaNs due to infinite momentum. See discussions in LHCb!1553 (merged) and Brunel!540 (merged).

Enabled by default, can be switched off with PrPixelTracking().AddQoverP = false. Default pT can be changed with PrPixelTracking().ptVelo, default is 400 MeV. Unlike TrackStateInitTool, there is no reverseVeloCharge property (to test tracking invariance under reversed magnetic field) since this can be mimicked by setting ptVelo negative.

@rquaglia @chasse @adudziak @fpolci @gdujany, @jonrob FYI. Please review and let me know what you think